Catalyst system for the reduction of NOx and NH3 emissions

ABSTRACT

This catalyst system simultaneously removes ammonia and enhances net NOx conversion by placing an NH 3 -SCR catalyst formulation downstream of a lean NOx trap. By doing so, the NH 3 -SCR catalyst adsorbs the ammonia from the upstream lean NOx trap generated during the rich pulses. The stored ammonia then reacts with the NOx emitted from the upstream lean NOx trap-enhancing the net NOx conversion rate significantly, while depleting the stored ammonia. By combining the lean NOx trap with the NH 3 -SCR catalyst, the system allows for the reduction or elimination of NH 3  and NOx slip, reduction in NOx spikes and thus an improved net NOx conversion during lean and rich operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/706,558, filed Feb. 16, 2010, which is a continuation of U.S.application Ser. No. 12/325,787 filed Dec. 1, 2008, now U.S. Pat. No.7,674,743 which is a continuation of U.S. application Ser. No.11/684,064 filed Mar. 9, 2007, now U.S. Pat. No. 7,485,273 which is acontinuation of U.S. application Ser. No. 10/065,470, filed Oct. 22,2002, now U.S. Pat. No. 7,332,135.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a catalyst system to facilitate thereduction of nitrogen oxides (NOx) and ammonia from an exhaust gas. Moreparticularly, the catalyst system of this invention includes a lean NOxtrap in combination with an ammonia selective catalytic reduction(NH₃-SCR) catalyst, which stores the ammonia formed in the lean NOx trapduring rich air/fuel operation and then reacts the stored ammonia withnitrogen oxides to improve NOx conversion to nitrogen when the engine isoperated under lean air/fuel ratios. In an alternate embodiment, athree-way catalyst is designed to produce desirable NH₃ emissions atstoichiometric conditions and thus reduce NOx and NH₃ emissions.

2. Background Art

Catalysts have long been used in the exhaust systems of automotivevehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides(NOx) produced during engine operation into non-polluting gases such ascarbon dioxide, water and nitrogen. As a result of increasinglystringent fuel economy and emissions standards for car and truckapplications, it is preferable to operate an engine under leanconditions to improve vehicle fuel efficiency and lower CO₂ emissions.Lean conditions have air/fuel ratios greater than the stoichiometricratio (an air/fuel ratio of 14.6), typically air/fuel ratios greaterthan 15. While lean operation improves fuel economy, operating underlean conditions increases the difficulty in treating some pollutinggases, especially NOx.

Regarding NOx reduction for diesel and lean burn gasoline engines inparticular, lean NOx adsorber (trap) technologies have been widely usedto reduce exhaust gas NOx emissions. Lean NOx adsorbers operate in acyclic fashion of lean and rich durations. The lean NOx trap functionsby adsorbing NOx when the engine is running under lean conditions-untilthe NOx trap reaches the effective storage limit-followed by NOxreduction when the engine is running under rich conditions.Alternatively, NOx reduction can proceed by simply injecting into theexhaust a sufficient amount of reductant that is independent of theengine operation. During this rich cycle, a short rich pulse ofreductants, carbon monoxide, hydrogen and hydrocarbons reduces the NOxadsorbed by the trap during the lean cycle. The reduction caused duringthe rich cycle purges the lean NOx adsorber, and the lean NOx adsorberis then immediately available for the next lean NOx storage/rich NOxreduction cycle. In general, poor NOx reduction is observed if the airexcess ratio λ is above 1. NOx reduction generally increases over leanNOx adsorbers as the λ ratio is decreased lower than 1. This air excessor lambda ratio is defined as the actual air/fuel ratio divided by thestoichiometric air/fuel ratio of the fuel used. The use of lean NOxadsorber (trap) technology, and in particular the rich pulse ofreductants, can cause the λ ratio to reach well below 1.

Lean NOx traps, however, often have the problem of low NOx conversion;that is, a high percentage of the NOx slips through the trap as NOx. NOxslip can occur either during the lean portion of the cycle or during therich portion. The lean NOx slip is often called “NOx breakthrough.” Itoccurs during extended lean operation and is related to saturation ofthe NOx trap capacity. The rich NOx slip is often called a “NOx spike.”It occurs during the short period in which the NOx trap transitions fromlean to rich and is related to the release of stored NOx withoutreduction. Test results depicted in FIG. 1a have shown that during thislean-rich transition, NOx spikes, the large peaks of unreacted NOxaccounts for approximately 73% of the total NOx emitted during theoperation of a lean NOx trap. NOx breakthrough accounts for theremaining 27% of the NOx emitted.

An additional problem with lean NOx traps arises as a result of thegeneration of ammonia by the lean NOx trap. As depicted in FIG. 1b ,ammonia is emitted into the atmosphere during rich pulses of the leanNOx adsorber. In laboratory reactor experiments, ammonia spikes as highas 600 ppm have been observed under typical lean NOx adsorber operation(see FIG. 1b ). While ammonia is currently not regulated, ammoniaemissions are being closely monitored by the U.S. EnvironmentalProtection Agency; and, therefore, reduction efforts must be underway.Ammonia is created when hydrogen or hydrogen bound to hydrocarbonsreacts with NOx over a precious metal, such as platinum. The potentialfor ammonia generation increases for a precious metal catalyst (such asa lean NOx trap) as the λ ratio is decreased, as the duration of therich pulse increases, and the temperature is decreased. There is thus anoptimum λ and rich pulse duration where the maximum NOx reduction isobserved without producing ammonia. Attempts to enhance conversion ofNOx by decreasing the λ ratio of the rich pulse duration leads tosignificant production of ammonia and thus results in high gross NOxconversion (NOx→N₂→NH₃), but much lower net NOx conversion (NOx→N₂).

In addition to nitrogen, a desirable non-polluting gas, and theundesirable NH₃ described above, N₂O is another NOx reduction products.Like NH₃, N₂O is generated over NOx adsorbers and emitted into theatmosphere during rich pulses. The gross NOx conversion is the percentof NOx that is reduced to N₂, N₂O and N₃. The net NOx conversion is thepercent of NOx that is reduced to nitrogen, N₂, only. Accordingly, thegross NOx conversion is equal to the net NOx conversion if nitrogen isthe only reaction product. However, the net NOx conversion is almostalways lower than the gross NOx conversion. Accordingly, a high grossNOx conversion does not completely correlate with the high portion ofNOx that is reduced to nitrogen.

The NOx conversion problem is magnified for diesel vehicles, whichrequire more than a 90% NOx conversion rate under the 2007 U.S. Tier IIBIN 5 emissions standards at temperatures as low as 200° C. While highNOx activity is possible at 200° C., it requires extreme measures suchas shortening the lean time, lengthening the rich purge time, andinvoking very rich air/fuel ratios. All three of these measures,however, result in the increased formation of NOx or ammonia.Accordingly, while it may be possible to achieve 90+% gross NOxconversion at 200° C., to date there has not been a viable solution toachieve 90+% net NOx conversion.

Accordingly, a need exists for a catalyst system that eliminates NOxbreakthrough during the lean operation as well has the NOx spikes duringthe lean-rich transition period. There is also a need for a catalystsystem that is capable of improving net NOx conversion. Finally, thereis a need for a catalyst system capable of reducing ammonia emissions.

SUMMARY OF THE INVENTION

This invention provides a solution for all of the above problems and, inparticular, reduces or eliminates ammonia emissions and improves the netNOx conversion of the catalyst system. These problems are solved bysimultaneously removing ammonia and enhancing NOx conversion with theuse of an NH₃-SCR catalyst placed downstream of the lean NOx adsorbercatalyst, as shown in FIG. 2. The NH₃-SCR catalyst system serves toadsorb the ammonia emissions from the upstream lean NOx adsorbercatalyst generated during the rich pulses. Accordingly, as shown in FIG.2, the ammonia emissions produced by the lean NOx adsorber is stored andeffectively controlled by the NH₃-SCR catalyst rather than beingemitted. This reservoir of adsorbed ammonia then reacts directly withthe NOx emitted from the upstream lean NOx adsorber. As a result, asshown in FIG. 3, the overall net conversion is enhanced from 55% to 80%,while depleting the stored ammonia, as a function of the SCR reaction:NH₃+NOx→N₂. The NH₃-SCR catalyst is then replenished with ammonia bysubsequent rich pulses over the lean NOx adsorber.

During the lean cycle for this lean NOx adsorber+NH₃-SCR system, the NOxbreakthrough from the upstream lean NOx adsorber is reduced continuouslyas it passes over the NH₃-SCR until the reservoir of ammonia isdepleted. In addition, during the rich cycle, large spikes of unreactedNOx, are created. The downstream NH₃-SCR catalyst thus serves to dampenthese large NOx, spikes by reacting the unreacted NOx, with thereservoir of stored ammonia emitted from the lean NOx adsorber. Ingeneral, the combination of the lean NOx, adsorber+NH₃-SCR catalystsystem allows for the reduction, or elimination, of ammonia emissionsand NOx slip, i.e., reduction of NOx breakthrough and NOx spikes and,therefore, improved net NOx conversion during lean and rich operation.

Additionally, under this invention, urea and/or ammonia does not need tobe injected into the exhaust system to effectuate the reaction betweenNOx and ammonia. Rather, the ammonia is automatically generated from theNOx present in the exhaust gas as it passes over the precious metal leanNOx adsorber during the rich pulses. The generated ammonia is thenstored on the downstream NH₃-SCR catalyst, to react with the unreactedNOx and thereby convert the unreacted NOx to nitrogen.

The NH₃-SCR catalyst thus serves to adsorb the ammonia from the upstreamlean NOx adsorber catalyst generated during the rich pulses. Under thissystem, the ammonia is stored and effectively controlled rather thanbeing emitted. This reservoir of adsorbed ammonia then reacts directlywith any NOx emitted from the upstream lean NOx adsorber. As a result,the overall net NOx conversion is enhanced from 55% to 80%, while theoverall gross NOx conversion is enhanced from 68% to 82%, as shown inFIG. 3.

In one alternative embodiment of this invention, the catalyst system canbe optimized and NOx reduction increased by vertically slicing the leanNOx trap and NH₃-SCR catalyst substrates to create separate catalystzones, such that the catalytic converter shell or can would havealternating sections of lean NOx trap and NH₃-SCR catalysts, as shown inFIGS. 4a, 4b and 4c . Under this embodiment, both technologies, the leanNOx trap formulation and the NH₃-SCR formulation, can be incorporatedinto a single substrate and/or a single converter can rather thanplacing the NH₃-SCR catalyst downstream of the lean NOx adsorber as twoseparate and distinct catalyst substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a graph illustrating the NOx spikes that occur during the NOxtrap lean-rich transition;

FIG. 1b is a graph illustrating NOx and NH₃ emissions from a typicalprior art lean NO_(x) adsorber system;

FIG. 2 depicts the lean NOx and NH₃-SCR catalyst system of the presentinvention;

FIG. 3 depicts reduced NOx emissions and NH₃ emissions as a result ofthe use of the lean NOx and NH₃-SCR catalyst system of the presentinvention, as shown in FIG. 2;

FIGS. 4a, 4b, and 4c depict three different zoned catalyst embodimentsof the lean NOx and NH₃-SCR catalyst system;

FIGS. 5a, 5b, and 5c provide graphs illustrating the reduced levels ofNOx and NH₃ emissions resulting from each of the three zoned catalystembodiments depicted in FIGS. 4a, 4b, and 4c at a 250° C. inlet gastemperature and operating at a 50 second lean cycle and 5 second richcycle;

FIGS. 6a, 6b and 6c provide graphs illustrating the reduced levels ofNOx and NH₃ emissions resulting from each of the three zoned catalystembodiments depicted in FIGS. 4a, 4b and 4c at a 200° C. inlet gastemperature and operating at a 25 second lean cycle and a 5 second richcycle;

FIGS. 7a, 7b and 7c show three proposed examples of washcoatconfigurations incorporating the lean NOx trap and NH₃-SCR formulationsinto the same substrate;

FIG. 8 is a graph illustrating the impact of NOx conversion afterhydrothermal aging; and

FIG. 9 depicts a modified three-way catalyst and NH₃-SCR catalyst systemof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this invention, net NOx conversion is improved and ammonia emissionsreduced through the use of a lean NOx trap and NH₃-SCR catalyst systemwhich operate together to produce and store ammonia and reduce NOx tonitrogen. In so doing, the catalyst system of the present inventionsolves three problems of lean NOx traps; namely, reducing NOxbreakthrough, NOx spikes and ammonia emissions.

In order to meet increasingly stringent fuel economy standards, it ispreferable to operate an automotive engine under lean conditions.However, while there is improvement in fuel economy, operating underlean conditions has increased the difficulty in reducing NOx emissions.As an example, for a traditional three-way catalyst, if the air/fuelratio is lean even by a small amount, NOx conversion drops to lowlevels. With traditional three-way catalysts, the air/fuel ratio must becontrolled carefully at stoichiometric conditions to maximize reductionof hydrocarbons, carbon monoxide and NOx.

Throughout this specification, NOx refers to nitrogen oxides, whichinclude nitrogen monoxide NO and nitrogen dioxide NO₂. Further, lean NOxadsorber and lean NOx trap are used interchangeably throughout thisspecification.

To achieve NOx reduction, under lean operating conditions, one option isthe inclusion of a lean NOx trap. While the lean NOx trap is generallyeffective in NOx reduction, lean NOx traps are known to have theproblems referred to as “NOx slip” which includes breakthrough of NOxduring the extended lean operation of the NOx trap and also NOx spikesgenerated during the transition from the lean to the rich cycle.

NOx spikes, or NOx emissions during the lean-rich transition, arebelieved to occur due to the exothermic heat generated from theoxidation of reductants, carbon monoxide, hydrocarbons and hydrogen, bythe oxygen released from the oxygen storage material—the temperaturerise can be as high as 80-100° C.

The problem of NOx spikes is illustrated in FIG. 1a , and the problem ofinsufficient net NOx conversion is illustrated in FIG. 1b . FIG. 1bdepicts laboratory reactor data of a lean NOx adsorber system operatingin an 85 second lean and 5 second rich cyclic pattern. The plot in FIG.1b shows the nitrogen species concentration as a function of time. Thelaboratory reactor data depicted in FIG. 1b resulted from a catalysthaving an engine swept volume (ESV) of 100%. Additionally, the reactorused to obtain the results in FIG. 1b was at a temperature of 300° C. Tobegin the cycle, 500 ppm of nitrogen oxide was fed into the reactorwhere much of it was stored during the 85 second lean duration. Duringthe 5 second rich duration, nitrogen oxide was reduced; however, asignificant amount of ammonia was formed. As illustrated in FIG. 1b ,the data shows ammonia spikes as high as 600 ppm under typical lean NOxadsorber operation. Conversion, however, is generally improved as the λratio is decreased during the rich pulse. Decreasing the λ ratio alsoleads to significant production of ammonia and thus results in highgross NOx conversion (NO_(x)→N₂+NH₃), but much lower net NOx conversion(NOx→N₂). As illustrated in FIG. 1b , the net NOx conversion to nitrogenfor this lean NOx adsorber system was only 55%.

Under the catalyst system of this invention, ammonia is reduced and thenet NOx conversion improved simultaneously by placing an NH₃-SCRcatalyst formulation downstream of the lean NOx adsorber catalyst, asshown in FIG. 2.

FIG. 2 is an illustration of the catalyst system of this invention,which is capable of simultaneously eliminating ammonia emissions andimproving net NOx conversion. As illustrated in FIG. 2, NOx producedduring engine operation is stored by the lean NOx adsorber during thelean cycle. Following the lean cycle, during the rich cycle of the leanNOx adsorber NOx is reduced and ammonia generated. The lean NOx adsorberstores much of the NOx during the lean operation and then reduces NOxduring rich pulses of the reductants. During the same rich pulses,significant amounts of ammonia are generated, as further illustrated inFIG. 1. As illustrated in FIG. 2, the lean NOx adsorber emits NO, NO₂,NH₃, and N₂O. These same gases then pass through the NH₃-SCR, where NH₃is stored. Accordingly, the addition of the NH₃-SCR catalyst downstreamallows for the adsorption of NH₃ and subsequent reaction with any NOxthat slips through the upstream lean NOx adsorber, which thus improvesthe overall net NOx conversion (NH₃+NO→N₂). As can be seen in FIG. 2,the catalyst system of this invention results in a significant net NOxconversion improvement, the elimination of ammonia emissions, and theproduction of non-polluting gases nitrogen and N₂O.

It should be noted that for diesel applications, lean NO, NOx adsorbersmust operate at lower temperatures compared to gasoline lean NOxadsorbers since the exhaust temperatures of diesel engines aresignificantly lower. More ammonia is generated at 200° C. than at 300°C. over lean NOx adsorbers and thus the catalyst system of thisinvention has an even greater potential for diesel applications.Likewise, the problem of NOx spikes is more critical at highertemperatures, the temperatures used for gasoline applications; and thusthe catalyst system of this invention is beneficial to control theunreacted NOx spikes that result from the operation of a lean NOxadsorber at operating temperatures typical for gasoline lean NOxadsorber applications.

The NH₃-SCR catalyst thus serves to adsorb the ammonia producednaturally from the upstream lean NOx adsorber catalyst generated duringthe rich pulses. As a result, the NH₃-SCR catalyst stores the ammonia,controlling it rather than allowing it to be emitted into theatmosphere. This reservoir of adsorbed NH₃ in the NH₃-SCR catalystreacts directly with the NOx emitted from the upstream lean NOx adsorber(trap).

In general, this invention works to clean NOx emissions—and thus hasapplicability for stationary sources as well as for moving vehicles.This invention may be used to reduce NOx emissions for nitric acidplants, or any other stationary source that requires the reduction ofNOx emissions. This invention is nonetheless particularly directed foruse with gasoline and diesel vehicles which, unlike stationary sources,have a wide range of operating parameters, especially temperatureparameters—which cannot be precisely controlled. The present inventionhas the ability to store large quantities of ammonia across a broadtemperature range to effectuate the reaction between ammonia andnitrogen oxides and thereby convert NOx to nitrogen.

As illustrated in FIG. 3, laboratory experiments have demonstrated thatthe use of a lean NOx adsorber plus NH₃-SCR catalyst system improves netNOx conversion from 55%, as illustrated in FIG. 1, to 80%. FIG. 3 is agraph displaying laboratory data obtained using the catalyst system ofthis invention, wherein NOx ppm are charted as a function of time. Asillustrated in FIG. 3, the catalyst system of this invention completelyeliminated the ammonia spikes created during the rich pulses of the leanNOx adsorber. In this system, ammonia is stored on the NH₃-SCR catalystwhere it reacts with NOx during the 85 second lean duration, which thusimproves the net NOx conversion from 55% to 80% with no additional fueleconomy penalty. As shown in FIG. 3, the improved net NOx conversion canbe observed by the much narrower profile-zero ppm NOx is emitted for asignificant amount of time as compared to the graph shown in FIG. 1 of asystem lacking the NH₃-SCR+lean NOx adsorber combination.

The reaction between the stored ammonia and NOx increases the overallnet NOx conversion, which is enhanced from 55%—the amount of NOxconverted in prior art lean NOx trap systems—to 80%—as a result of thecombination of a lean NOx trap and NH₃-SCR catalyst system. Moreover, inaddition to improving net NOx conversion, the ammonia stored in theNH₃-SCR catalyst is depleted during the SCR reaction wherein ammonia andnitrogen oxide are reacted to produce nitrogen. The NH₃-SCR catalyst isreplenished with ammonia by subsequent rich pulses over the lean NOxadsorber that causes a portion of the NOx to react with hydrogen to formammonia.

It should be noted that no urea or ammonia needs to be injected into theexhaust system to effectuate the reaction between ammonia and NOx.Rather, the ammonia is naturally generated from the NOx present in theexhaust gas as it passes over the lean NOx trap during rich pulses. Morespecifically, ammonia is naturally created during the fuel rich cycle ofthe lean NOx trap. Ammonia is naturally produced as it passes over theprecious metal active component of the lean NOx trap. Similarly, theammonia could also be generated in a conventional precious metal basedTWC located upstream of a LNT/NH₃-SCR system.

For this invention, the lean NOx trap is optimized for ammoniageneration by removing oxygen storage capacity (OSC) and therebyenhancing the rich cycle and thus creating a greater quantity of ammoniafor reaction with the NOx in the downstream NH₃-SCR catalyst. In apreferred embodiment, the lean NOx trap includes platinum as theprecious metal. Platinum is the preferred precious metal because it isbelieved that a greater quantity of NH₃ is produced over platinum thanrhodium, palladium and/or a combination of the precious metals.Nonetheless, other precious metals such as palladium and rhodium, andthe combination of one or more of the precious metals platinum,palladium and rhodium may also be used to generate NH₃.

Additionally, the lean NOx trap of this invention preferably includes a“VNOx adsorbing material” or NOx storage component/material, which canbe alkali and alkali earth metals such as barium, cesium, and/or rareearth metals such as cerium and/or a composite of cerium and zirconium.Although an alternative catalyst formulation that does not contain a NOxstorage component but generates ammonia from NOx, may also be utilized,in the most preferred embodiment, the NOx storage material should havethe ability to store NOx at low temperature ranges, specifically in therange of 150° C.-300° C. The NH₃ thermodynamic equilibrium under richconditions is maximized during the temperature range of 150° C.-300° C.

In general, to increase the NOx storage function of the lean NOx trapand effectuate the NOx conversion reaction, in the preferred embodiment,the lean NOx trap has the following characteristics: (1) the inclusionof platinum as the precious metal; (2) the ability to store NOx between150° C. and 500° C. during the lean portion of the cycle; (3) theability to maximize the duration of the lean NOx trap rich cycle; (4)the ability to generate ammonia at the 150° C.-500° C. temperaturerange; (5) minimize OSC to lessen fuel penalty; and (6) lower λ togenerate more ammonia. Ammonia production is maximized at the preferredtemperature range, 150° C.-300° C.—which also correlates with the steadystate equilibrium range for ammonia creation. It bears emphasis thatother NOx storage components may be utilized, especially for stationarysources, where sulfur poisoning does not pose a threat.

Most simply, the NH₃-SCR catalyst may consist of any material orcombination of materials that can adsorb ammonia and facilitate theNOx+NH₃ to yield nitrogen. The NH₃-SCR catalyst should preferably bemade of a base metal catalyst on a high surface area support such asalumina, silica, titania, zeolite or a combination of these. Morepreferably, the NH₃-SCR catalyst should be made of a base metal selectedfrom the group consisting of Cu, Fe, and Ce and/or a combination ofthese metals, although other base metals may be used. Base metalsgenerally are able to effectuate NOx conversion using ammonia while boththe base metals and the high surface support material serves to storeNH₃. The base metal and high surface area support such as zeoliteselected should preferably be one that can store NH₃ over the widestpossible temperature range. Likewise, the base metal selected ispreferably one that can convert NO and NO₂ to N₂ across the widestpossible temperature range and the widest range of NO/NO₂ ratios.

The advantage of the catalyst system of this invention is the use of acombination of a lean NOx trap and an NH₃-SCR catalyst. The use of alean NOx trap in the present system allows for much greater storage ofNOx R, because the NOx breakthrough that would otherwise happen can becontrolled by the NH₃-SCR catalyst. Additionally, the use of a lean NOxtrap as part of this system allows for the operation of the engine atlean conditions for a longer time and thus provides improved fueleconomy. If, for example, a three-way catalyst is used as the NOxstorage mechanism, NOx storage is significantly limited, as well as theproduction of ammonia. To maximize the reduction of emissions, athree-way catalyst must be operated at stoichiometric conditions.Accordingly, unless the three-way catalyst is run on the rich side 100%of the time, ammonia production is significantly less than for a typicallean NOx trap. As set forth above, the efficiency of a three-waycatalyst is compromised if it is operated at conditions other than atstoichiometric conditions. Thus, the combination of a lean NOx trap andNH₃-SCR catalyst allows for significant NOx storage and ammoniaproduction and thus increases net NOx conversion.

In a preferred embodiment, the lean NOx trap and NH₃-SCR catalystconstitute alternating zones in a single substrate and/or a singlecatalytic converter can. This zoned design, as shown in three differentembodiments in FIGS. 4a-4c , is believed to maximize the reactionbetween ammonia and NOx.

As illustrated in FIG. 4, three zoned catalyst system embodiments wereevaluated on a laboratory flow reactor. The total catalyst systemdimensions were held constant at a 1″ diameter and 2″ length. The firstsystem, labeled “4 a”, had a 1″ long lean NOx trap followed by a 1″ longNH₃-SCR catalyst. In the second system, labeled “4 b”, the catalystsamples were sliced in half to yield alternating ½″ long sections.Finally, in the third system, labeled “4 c”, the same catalyst sampleswere further cut in half to yield ¼″ long sections, again of the leanNOx trap and NH₃-SCR catalyst technologies. It should be noted that eachtime the catalysts were sliced, as shown in “4 b” and “4 c”, the overalllength of the catalyst system was reduced slightly, approximately 3/16″total. The alternating lean NOx trap and NH₃-SCR catalyst zones can becreated in a single substrate or the lean NOx trap and NH₃-SCR catalystprepared, cut as desired and then placed adjacent one another in asingle can. The zones are preferably formed in a single substrate.However, cut substrates placed in alternating fashion also exhibitimproved net NOx conversion.

Under the zoned catalyst designs shown in FIGS. 4a-4c , wherealternating lean NOx and NH₃-SCR catalyst zones are provided, theammonia formed by the lean NOx trap is believed to be immediatelyadsorbed by the NH₃-SCR catalyst for use in the NOx conversion reaction.It is further believed that the greater the separation between the leanNOx trap and the NH₃-SCR catalyst, the greater chance there is for theammonia to be converted back into NOx. It is further believed thatoxygen is more abundant in the back of a catalyst substrate and thus theoxygen may be available to effectuate the unwanted conversion of theammonia back to nitrogen oxide. Accordingly, if the catalyst substrateis too long, there may be some undesired conversion that takes place;and thus in a preferred embodiment, the substrate is designed so thatammonia is available for immediate reaction with NOx.

FIGS. 5a-5c illustrate laboratory reactor data of the three differentzoned catalyst system embodiments shown in FIGS. 4a-4c . This laboratorydata was obtained with the three catalyst systems operating at a 250° C.inlet gas temperature and operating with 50 second lean and 5 secondrich cycles. Additionally, the inlet concentration of the NOx feed gaswas 500 ppm and the overall space velocity was 15,000 per hour. Asillustrated in FIGS. 5a-5c , with the use of a two-zoned catalyst systemas depicted in FIG. 5a , approximately 50 ppm of NO is emitted. Thistwo-zone catalyst system resulted in a gross NOx conversion of 95% and anet NOx conversion of 66%. The four-zone catalyst embodiment, depictedas FIG. 5b , significantly reduced NOx emissions, well below the 15 ppmrange, to result in gross NOx conversion of 99% and a net NOx conversionof 86%. Finally, as illustrated by the eight zoned catalyst embodiment,FIG. 5c , gross NOx conversion is 100% and net NOx conversion is 97.5%.The improvement comes from the reduction of N₂O elimination of the NH₃breakthrough and reduction of NOx. Accordingly, as the catalyst systemis zoned down from 1″ sections to ¼″ sections, the test results revealedan associated improvement in net NOx conversion.

As shown in FIGS. 5a-5c , a zoned catalyst, with alternating lean NOxand NH₃-SCR catalysts in 1″ to ¼″ sections significantly improves thenet NOx conversion from 66% to 97.5%. In addition, the gross NOxconversion is improved from 95% to 100%. In general, the improvement inthe net NOx conversion is the function of the elimination of the ammoniaslip, reduction in N₂O, and extra NOx reduction related to the NH₃+NOxreaction on the NH₃-SCR catalyst. It is further believed that the dropin N₂O emissions is likely due to a higher fraction of the NOx reductionreaction proceeding on the NH₃-SCR catalyst rather than the lean NOxtrap. NOx reduction over a platinum-containing-lean NOx trap results inhigh levels of N₂O generation, whereas the NH₃-SCR catalyst has a highselectivity to nitrogen.

FIGS. 6a-6c depicts laboratory data obtained using the three-zonedcatalyst embodiments originally shown in FIGS. 4a-4c at a 200° C. inletgas temperature operating with a 25 second lean cycle and a 5 secondrich cycle. As compared to FIGS. 5a-5c , it should be noted thatshortening the lean time from 50 seconds, as used in FIGS. 5a-5c , to 25seconds, resulted in a substantial higher steady emission of ammonia—afact which results in reduced net NOx conversion rates, as compared tothe data charted in FIGS. 5a-5c . As can be seen in FIGS. 6a-6c , theuse of smaller zoned sections from two zones to eight zones and thus 1″sections down to ¼″ sections, as illustrated in FIGS. 6a and 6c ,improves the net NOx conversion from 50% to 81%. Again, this improvementis believed to come mainly from the reduction of ammonia breakthroughand a small reduction in N₂O emissions. This lab data was obtained withan inlet concentration of the NOx feed gas at 500 ppm and an overallspace velocity at 15,000 per hour.

As set forth above, in the preferred embodiment, the lean NOx trapwashcoat and NH₃-SCR washcoat are combined in a single substrate ratherthan placing the NH₃-SCR formulation downstream of the lean NOx adsorberas two separate catalyst substrates. Under this embodiment, the catalystformulations can be incorporated together by mixing or layering thewashcoats on a substrate.

FIGS. 7a-7c show three proposed washcoat configurations incorporatingthe lean NOx trap and NH₃-SCR formulations into the same substrate. Asshown in FIGS. 7a and 7b , the first and second proposed configurationshave the lean NOx trap and NH₃-SCR washcoat formulations on the bottomand top layer, respectively. It is believed that the top layer could bea highly porous structure that allows better and faster contact betweenthe chemicals and gas phase and the active sites in the second layer.The third configuration, as shown in FIG. 7c , involves the use of a onelayer washcoat containing both lean NOx trap and NH₃-SCR washcoatformulations. Under this third configuration, shown in FIG. 7c , thewashcoat composition of the lean NOx trap and NH₃-SCR catalyst could behomogeneously or heterogeneously mixed. For a heterogeneously mixedcomposition, the formulation of the lean NOx trap and NH₃-SCR catalystare separated. However, they contact each other in varying degrees bycontrolling the size of the grain structures. The homogeneously mixedcomposition allows for a more intimate contact between the twoformulations and is thus preferred.

The invention also contemplates engineering such combinations within thepores of the monolithic substrate. An example of this is incorporatingwashcoat into porous substrates used for filtering diesel particulatematter. Thus, this lean NOx trap/NH₃-SCR catalyst concept can beintegrated into diesel particulate matter devices.

This very active SCR reaction of NOx and ammonia proceeds with orwithout oxygen present. Koebel et al. reports that the fastest SCRreaction involves equal molar amounts of NO and NO₂ NO and NO₂ thenreact with two NH₃ to yield N₂ in the absence of oxygen. In contrast,the lean NOx adsorber reaction of NOx plus CO is highly reactive only inan oxygen-free environment. In a lean NOx adsorber system, NOx isadsorbed during the lean cycle duration, NOx is not reduced.Accordingly, NOx reduction is limited to only the rich pulse timeduration. On the other hand, the lean NOx adsorber+NH₃-SCR catalystsystem allows for NOx reduction reaction to proceed during both the leanand rich time durations. Accordingly, ammonia as a reductant can beconsidered as a much more robust reductant than carbon monoxide.

As set forth above, the fastest SCR reaction involves equal molaramounts of NO and NO₂ Accordingly, FIG. 8 illustrates the impact ofvarying NO:NO₂ ratios after hydrothermal aging. FIG. 8 is a graph ofthree NH₃-SCR catalyst formulations over a wide NO:NO₂ range. In thelaboratory, it was possible to control the NO:NO₂ ratio entering thedownstream NH₃-SCR catalyst. Accordingly, the NO:NO₂ ratio entering theNH₃-SCR catalyst was solely dependent on the upstream lean NOx adsorber.In some cases, the majority of the feed NOx (especially NOx spikes) aremade up of mostly NO rather than NO₂ Accordingly, it is believed thatthe catalyst formulations of this invention will enhance reported netNOx efficiency—and thus the preferred catalyst is one that is capable ofoperating across the broadest range of NO:NO₂ ratios, and at a fullspectrum of temperature ranges.

In general, since NH₃-SCR catalysts do not contain precious metals, theyare significantly less costly than a typical lean NOx trap. Accordingly,it is more cost effective to have an overall catalyst system containinga lean NOx trap adsorber and an NH₃-SCR catalyst system, rather than onethat uses two lean NOx trap adsorbers. Additionally, the incorporationof both a lean NOx trap and NH₃-SCR washcoat into a single substratewill significantly reduce substrate costs.

In another embodiment of this invention, NH₃ and NOx in an exhauststream are reduced using a stoichiometric three-way catalyst system.This three-way catalyst system has particular application for highspeed/high flow rate conditions (i.e., US06 conditions). Currently,three three-way catalysts are used for such high speed conditionapplications, wherein the third three-way catalyst is primarily directedto NOx removal for high speed/high flow rate conditions. Under thisalternate embodiment, the third three-way catalyst can be substitutedwith an NH₃-SCR catalyst to store NH₃ for reaction with NOx to improvenet NOx conversion, eliminate NH₃ emissions and reduce catalyst costs.

To improve net NOx and NH₃ reduction, the second three-way catalyst canbe modified to enhance the three-way catalyst's ability to generate NH₃emissions. To this end, in a preferred embodiment, the three-waycatalyst is designed to generate desirable NH₃ creation by usingplatinum as the precious metal of the three-way catalyst, by placingplatinum on the outer layer of the three-way catalyst to maximize theNO+H₂—NH₃ reaction. Likewise, the oxygen storage capacity (OSC) of thethree-way catalyst can be removed to further promote the creation of“desirable” NH₃. By doing so, the NH₃ purposely generated during richoperation can then be stored by the NH₃-SCR catalyst for subsequentreaction with NOx emissions, and thereby control both NOx and NH₃emissions under all operating conditions.

When a car is operated under rich conditions, the air/fuel ratio is lessthan 14.6, hydrogen is produced in the exhaust via the water-gas shiftreaction: CO+H₂O→CO₂+H₂. The hydrogen that is produced then reacts withNOx, as it passes over the precious metal surface to create “desirable”ammonia. The ammonia produced is then stored on an NH₃-SCR catalyst tohelp reduce net NOx conversion. The reaction of NOx+NH₃→N₂+H₂O can thentake place on a separate NH₃ selective catalyst, capable of convertingNO₂ and NO to N₂.

As shown in FIG. 9, a stoichiometric three-way catalyst/NH₃-SCR catalystsystem 10 is depicted, including a first three-way catalyst 14positioned in close proximity to the engine 12 to reduce cold startemissions. The second three-way catalyst 16 is modified as describedabove to enhance the ability of the second three-way catalyst 16 togenerate NH₃ emissions. Downstream of the second three-way catalyst 16is an NH₃-SCR catalyst 18 that functions to store NH₃ produced by themodified second three-way catalyst 16 for reaction with NOx emissions,to reduce both NOx and NH₃ emissions.

By substituting the third three-way catalyst as currently used with anNH₃-SCR catalyst and thereby eliminating the need for a third preciousmetal containing catalyst, significant cost savings can be achieved.

It should further be noted that this invention also contemplates the useof a three-way catalyst, in combination with a lean NOx trap and anNH₃-SCR catalyst.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

The invention claimed is:
 1. A method for reducing pollutants in theexhaust gas of an engine having a system including a nitrogen oxideadsorber and a NH₃-SCR catalyst downstream of the nitrogen oxideadsorber, comprising the steps of: supplying the nitrogen oxide adsorberwith exhaust gas while the engine operates under lean conditions,wherein the exhaust gas is in a lean exhaust gas; transitioning theengine operations from lean to rich conditions; supplying the nitrogenoxide adsorber with exhaust gas during said transitioning; minimizingthe oxygen content of the nitrogen oxide adsorber prior to richconditions to facilitate the reduction of NOx to NH₃; completing thetransition from lean to rich conditions; and supplying the nitrogenoxide adsorber with exhaust gas while the engine operates under richconditions, wherein the exhaust gas is a rich exhaust gas.
 2. A methodfor reducing NOx and NH₃ in exhaust gas, comprising the steps of:supplying exhaust gas from an engine to a lean NOx trap and an SCRcatalyst located downstream of said lean NOx trap while said engineoperates in a lean cycle; supplying exhaust gas from said engine to thelean NOx trap and the SCR catalyst while said engine operations aretransitioned from a lean cycle to a rich cycle; transitioning from alean cycle to a rich cycle; and supplying exhaust gas from said engineto the lean NOx trap and the SCR catalyst while said engine operates ina rich cycle, wherein the oxygen content of the lean NOx trap isminimized to facilitate the reduction of NOx to NH₃.
 3. The method ofclaim 2, wherein the oxygen content of the lean NOx trap is minimizedprior to a rich cycle.
 4. A method for improving NOx conversion from theexhaust gas of an internal combustion engine, comprising the steps of:operating an engine under a lean cycle wherein a lean exhaust gasincludes NOx; storing NOx from the lean exhaust gas in a lean NOx trapplaced downstream of said engine; transitioning the operation of theengine to a rich cycle and supplying exhaust gas to said lean NOx trapduring said transition; completing the transition to a rich cyclewherein the engine supplies a rich exhaust gas and said rich exhaust gasreacts with said stored NOx in said lean NOx trap to produce reactionproducts comprising ammonia; storing said ammonia in an SCR locateddownstream of said lean NOx trap, wherein said stored ammonia reactswith NOx that passes through said lean NOx trap and the oxygen contentof the lean NOx trap is minimized to facilitate the reduction of NOx toNH₃.
 5. The method of claim 4, wherein the oxygen content of the leanNOx trap is minimized prior to the rich cycle.
 6. A method for purifyingthe exhaust gas from an engine, comprising: supplying a zoned catalystsystem with a lean exhaust gas, wherein said zoned catalyst systemcomprises a first zone of lean NOx trap and a second zone of SCRcatalyst downstream of the first zone; supplying the zoned catalystsystem with a rich exhaust gas; transitioning between said lean exhaustgas supplying step and said rich exhaust gas supplying step, whereinexhaust gas is supplied to said zoned catalyst system during saidtransition; and completing said transition from said lean exhaust gassupplying step to said rich exhaust gas supplying step.
 7. A method ofclaim 6, wherein said zoned catalyst system further comprises a thirdzone of lean NOx trap and fourth zone of SCR catalyst downstream of thefirst and second zones.
 8. A method of claim 6, wherein said zonedcatalyst system is created in a single substrate.
 9. A method of claim6, wherein said zoned catalyst system is created from cut catalystcompositions placed adjacent to one another in a single can.
 10. Themethod of claim 6, wherein the oxygen content of the lean NOx trap zoneis minimized to facilitate the reduction of NOx to NH₃.
 11. The methodof claim 10, wherein the oxygen content of the lean NOx trap zone isminimized prior to said rich exhaust gas supplying step.